Position sensor using fiber bragg gratings to measure axial and rotational movement

ABSTRACT

A sensor is disclosed herein. The sensor includes a fiber operable to communicate a light wave. The sensor also includes at least first and second Fiber Bragg Gratings disposed along the fiber. The sensor also includes a structure operable to be deformed in a plane of deformation. The at least first and second Fiber Bragg Gratings are disposed on opposite sides of the structure in the plane of deformation. The sensor also includes an interrogation unit operable to receive first and second signals corresponding to first and second wavelengths from the at least first and second Fiber Bragg Gratings. The first signal is associated with the first Fiber Bragg Grating and the second signal is associated with the second Fiber Bragg Grating. The sensor also includes a processor operable to derive a difference between the wavelengths of the first and second signals and compare the difference with data correlating wavelength differences to extents of deformation of the structure to yield a current extent of deformation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/522,930 for a POSITION SENSOR USING FIBER BRAGGGRATINGS TO MEASURE AXIAL AND ROTATIONAL MOVEMENT, filed on Aug. 12,2011, and is a divisional application of U.S. Utility patent applicationSer. No. 13/584,776 for a POSITION SENSOR USING FIBER BRAGG GRATINGS TOMEASURE AXIAL AND ROTATIONAL MOVEMENT, filed on Aug. 13, 2012, both ofwhich are hereby incorporated by reference in their entireties.

BACKGROUND

1. Field

The invention relates to a fiber optic sensor.

2. Description of Related Prior Art

It is known to use sensors to detect strain in a structure.

SUMMARY

In summary, the invention is a sensor. The sensor includes a fiberoperable to communicate a light wave. The sensor also includes at leastfirst and second Fiber Bragg Gratings disposed along the fiber. Thesensor also includes a structure operable to be deformed in a plane ofdeformation. The at least first and second Fiber Bragg Gratings aredisposed on opposite sides of the structure in the plane of deformation.The sensor also includes an interrogation unit operable to receive firstand second signals corresponding to first and second wavelengths fromthe at least first and second Fiber Bragg Gratings. The first signal isassociated with the first Fiber Bragg Grating and the second signal isassociated with the second Fiber Bragg Grating. The sensor also includesa processor operable to derive a difference between the wavelengths ofthe first and second signals and compare the difference with datacorrelating wavelength differences to extents of deformation of thestructure to yield a current extent of deformation.

BRIEF DESCRIPTION OF THE DRAWINGS

Advantages of the present invention will be readily appreciated as thesame becomes better understood by reference to the following detaileddescription when considered in connection with the accompanying drawingswherein:

FIG. 1 is a perspective view of a first exemplary embodiment of theinvention;

FIG. 2 is a front view of a second exemplary embodiment of the inventionwith an upper-right portion cut-away;

FIG. 3 is a partial cross-section taken through section lines 3-3 inFIG. 2;

FIG. 4 is a magnified detail view of the detail circle 4 in FIG. 2;

FIG. 5 is a graph correlating output of Fiber Bragg Gratings totemperature;

FIG. 6 is a graph displaying the differential output of a plurality ofFiber Bragg Gratings relative to an extent of deformation of a structureat a first temperature;

FIG. 7 is a graph displaying the differential output of a plurality ofFiber Bragg Gratings relative to an extent of deformation of a structureat a second temperature;

FIG. 8 is a graph displaying the quotient of outputs of a plurality ofFiber Bragg Gratings relative to an extent of deformation of a structureat a first temperature;

FIG. 9 is a graph displaying the quotient of outputs of a plurality ofFiber Bragg Gratings relative to an extent of deformation of a structureat a second temperature; and

FIG. 10 is a graph displaying the wavelength of outputs of a Fiber BraggGrating relative to an extent of deformation, with curves for currenttemperature and for a reference or known temperature.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

The invention, as demonstrated by the exemplary embodiments describedbelow, provides an apparatus and method to measure rotational or lineardisplacement and temperature using Fiber Bragg Gratings (FBG). In oneembodiment, the translation or rotational displacement of a shaft, withan integral wheel and screw, is converted to the bending of a resilientmember. The proportional strain (compressive and tensile) induced by thebending can then be measured by two FBGs. In another embodiment, thedisplacement of a resilient member is induced by a spiral shaft with anintegral cam is detected. In various embodiments, the FBGs are fixed toopposing sides of the resilient member such that one experiences tensilestrain while the other experiences compressive strain. The design of theexemplary embodiments disclosed below enables accurate displacementmeasurements while also measuring, and compensating for, any temperaturerelated effects to the sensors.

FIG. 1 shows a first exemplary embodiment of the invention. An actuationrod 12 is supported in an outer housing by two linear bearings. The rod12 contacts a wheel 14 and linear movement of the rod 12 results inrotational movement of the wheel 14. The wheel 14 is fixedly mounted ona threaded axle 16 and movement of the wheel 14 results in movement ofthe axle 16 as well. As the threaded axle 16 rotates, a forcing block18, which has threads that mate with the threads of the axle 16, movesalong an axis referenced at 20. When the axle 16 rotates in a firstangular direction referenced at 22, the block 18 can move in a firstdirection referenced at 24 along the axis 20. When the axle 16 rotatesin a second angular direction opposite to the first angular direction,the block 18 can move in a second direction along the axis 20 oppositeto the first direction.

It is noted that the block 18 is generally mounted on a rail 36. Therail 36 is received in a notch 38 defined by the rail 36. Engagementbetween the notch 38 and the rail 36 limits movement of the block 18along the axis 20 but does not prevent all movement.

Movement of the forcing block 18 in the first direction 24 imparts aload on a spring member 26. Distal ends (one referenced at 28 and theother hidden) defined by a pair of arms 40, 42 of the spring member 26are elastically deformed in the first direction 24 relative to a baseportion 30 of the spring member 26 when the block 18 moves in the firstdirection 24. The arms 40, 42 project from the base portion 30. As theforcing block 18 displaces the distal ends of the spring member 26relative to the base portion 30, strain is created along a length of thespring member 26 between the base portion 30 and the distal ends, in thearms 40, 42.

Two Fiber Bragg Gratings (hereafter FBG) are attached to the springmember 26 to sense conditions that can be electronically communicated,measured, and correlated to the strain in the spring member 26, as wellas correlated to the extent of movement of the block 18, the wheel 14,and the rod 12. A first FBG 32 is attached to the first arm 40 of thespring member 26 to sense conditions corresponding to compressivestrain. A second FBG 34 is attached to the second arm 42 to measuretensile strain.

The FBGs 32, 34 are in electronic communication with an interrogationunit (referenced schematically at 44) through the fibers 46, 48. Anelectronic processor 45 can be integral with or separate from theinterrogation unit 44. The processor 45 can process the signals receivedfrom the FBGs 32, 34. Each fiber 46, 48 is operable to communicate alight wave and each can extend at least partially through a sheath, suchas sheath 50. It is noted that the fiber 46, 48 are integral with oneanother and also with loop portion 52, to define a continuous waveguide. As temperature affects the wavelength of a FBG, it is difficultto differentiate between wavelength change due to physical strain andthe change 20 induced by thermal strain. The use of two FBGs in theexemplary embodiment of the invention allows for temperaturecompensation in strain measurement. By finding the difference betweenthe wavelength changes arising from FBGs 32 and 34, the effect ofthermal strains can be cancel, leaving only the strain due to mechanicaldeformation. The resultant strain is an accurate representation of thetrue strain in the spring member 26. This strain can then be scaled intothe desired engineering units of measure.

Deriving the differential wavelength as set forth above also revealsthermal strain. The “cancelled” portion of strain corresponds to thetemperature calibration of either of the FBGs 32 or 34. Thus, thetemperature of either FBG 32 or 34 can be calculated by detracting theknown mechanical strain. This allows the embodiment of the invention tomeasure both a position of one of the structural components (derivedfrom strain) and the temperature.

FIG. 2 is a planar view of a second embodiment of the invention with aportion cut-away. An actuation tube 54 can be supported by an outer tube56. The tubes 54, 56 can be concentric. As shown in FIG. 3, internal tothe actuation tube 54, two bearings support an internal precision spiraltransfer shaft 58. A nut 60 encircles the spiral transfer shaft 58 andis fixed to the actuation tube 54. The nut 60 forces the spiral transfershaft 58 to rotate in response to linear movement of the actuation tube54. Rotation of the spiral transfer shaft 58 causes rotational movementof a cam 62. As the cam 62 rotates it applies a load to a spring member64, causing the spring member 64 to bend. As the cam 62 displaces a tip66 (referenced in FIG. 2) of the spring member 64, strain is createdalong a length of the spring member 64 between the tip 66 and a baseportion 68.

FIG. 4 is taken in a plane of deformation of the spring member 64; thedeformation of the spring member 64 is visible in this plane. First andsecond FBGs 70, 72 are attached to the spring member 64. The first FBG70 can be attached to a top of the spring member for sensing conditionscorresponding to compressive strain as the spring member 64 is deflectedaway, upward (relative to the perspective of FIG. 2) by the cam 62. Thesecond FBG 72 can be attached to bottom of the spring member 64 to senseconditions corresponding to tensile strain as the spring member 64 isdeflected upward by the cam 62.

As with the first embodiment, the FBGs 70, 72 can be connected to aninterrogation unit. Also, the operation of the second embodiment issimilar to the operation of the first embodiment in that the use of twoFBGs allows for temperature compensation in strain measurement. Thisallows the second embodiment of the invention to measure both positionand temperature.

The method of measuring strain will now be described. In FIG. 5, theoutputs of the two FBGs for an embodiment of the invention are shown forthree separate temperatures as a function of the sensor position. Eachcurve (the straight lines in the graph of FIG. 5 are designated hereinas curves) represents an extent of deformation of the structure beingmonitored. The two FBGs are distinguished from one another by thedesignations “A” and “B.” The horizontal axis defines the position ofthe sensor as a percentage and corresponds to a range over which thespring member is expected to deform in a particular operatingenvironment. In other words, at 50% for example, the spring member willhave deformed approximately 50% of the maximum amount the spring memberis expected to possibly deform. Thus, the position of sensor isanalogous to the extent of deformation of the structure being sensed, aspring member in the exemplary embodiments.

The graph of FIG. 5 reveals that as the temperature of the FBGsincrease, the outputs of the FBGs (in wavelength) also increase. Sincethe common increase in wavelength due to temperature affects the outputof both FBGs, taking the difference of the two outputs or taking a ratioof the two outputs allows for a cancelation of thermal effects on themeasurement of position change, leaving only the change due tomechanical strain.

FIGS. 6 and 7 correlates the difference in wavelengths between FBGs Aand B with the position of sensor. FIGS. 6 and 7 also show thedifference between exemplary FBGs A and an FBG B when the pairs of FBGsare at two different temperatures. In FIG. 6 the FBGs A and B are at 10°C. and in FIG. 7 the FBGs A and B are at 38° C. A comparison between thetwo graphs shows that the differential output at different temperaturesyields the same position of sensor for either temperature. In otherwords, the graphs of FIGS. 6 and 7 show that mechanical strain can beaccurately determined regardless of temperature.

FIGS. 8 and 9 are analogous to FIGS. 6 and 7. FIGS. 8 and 9alternatively show the quotient of a FBG A and a FBG B with at twodifferent temperatures, each FBG having the same temperature. As shownin FIGS. 8 and 9, the calculated difference between the outputs of theFBGs A and B at the different temperatures yields the same position ofsensor at either temperature.

Example 1

A point 74 referenced on the graph of FIG. 5 corresponds to the FBG A at10° C. and at 10% of the position of sensor. A point 76 referenced onthe graph of FIG. 5 corresponds to the FBG B at 10° C. and at 10% of theposition of sensor. The coordinates of point 74 are (10%, 1558.6nanometers) and the coordinates of point 76 are (10%, 1559.6nanometers). The vertical, differential distance between points 74 and76 is 1 nanometer. This value is confirmed by reference to FIG. 6, inwhich coordinates of point 78 are (10%, 1 nanometer). The quotient ofthe wavelength values (1559.6 divided by 1558.6) is equal to 1.0006.This value is confirmed by reference to FIG. 8, in which coordinates ofpoint 80 are (10%, 1.0006).

It is noted that the value of the position of sensor would be the valuebeing pursued. After the differential wavelength or quotient is known,FIG. 6 or 8 would be consulted to derive the position of sensor. In anembodiment of the invention, the data graphically shown in FIGS. 6 and 8could be in the form of a table stored in the memory of an electronicprocessor. An electronic processor can receive the signal inputs fromthe FBGs, determine the differential wavelength and/or quotient, accessa table of data analogous to the data in FIG. 6 or 8, and obtain theposition of sensor. An electronic processor in an embodiment of theinvention can be component of the interrogation unit.

Once the position of the sensor is calculated the temperature of thesensor can be determined. The measured output of one of the FBGs at theknown position can be referenced against known output for a FBG at thesame position and at a known temperature. The dashed line in FIG. 10represents the output of an FBG at an unknown temperature. The solidline in FIG. 10 represents the output of an FBG at a known temperature.Data associated with FBG output at one or more known temperatures can bestored as data in an electronic processor that receives and processessignals from the FBGs. FIG. 5 shows a plurality of curves/linesrepresenting observed FBG output at various temperatures; an electronicprocessor can retain such data in memory.

Example 2

The observed output of an FBG is referenced at point 82 in FIG. 10. Ithas been previously determined that the position of sensor is 50%.Several alternative methods can be applied to derive the temperature ofthe FBG. In one embodiment of the invention, the vertical position ofthe point 82 relative to other, known curves can be the basis ofinterpolation. For example, if the point 82 were vertically equidistantbetween a curve associated with 0° C. and a curve associated with 20°C., the temperature of the FBG could be determined to be 10° C. if therelationship between the 0° C. curve and the 20° C. curve was known tobe parallel. Alternatively, the difference in wavelength can corresponddirectly to the temperature difference. In FIG. 10, the point 82 isapproximately 0.4 nanometers vertically distance from a point 84 on 0°C. curve. In an embodiment of the invention, the distance 0.4 nanometerscan correspond to a 40° C. temperature difference. The FBG operating atpoint 82 would thus be operating at a temperature of 40° C.

Embodiments of the invention can be applied to methods and apparatusrelated to monitoring the position and temperature of mechanicalcomponents such as, by way of example and not limitation, variable valvepositions, actuator stroke length, flow control devices, inlet guidevane positions, automation feedback loops, thermal growth of structures,gate position and component deflection.

While the invention has been described with reference to an exemplaryembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims. Further, the “invention” as that term is used in this documentis what is claimed in the claims of this document. The right to claimelements and/or sub-combinations that are disclosed herein as otherinventions in other patent documents is hereby unconditionally reserved.

What is claimed is:
 1. A sensor comprising: a fiber operable tocommunicate a light wave; at least first and second Fiber Bragg Gratingsdisposed along said fiber; a structure operable to be deformed in aplane of deformation, wherein said at least first and second Fiber BraggGratings are disposed on opposite sides of said structure in said planeof deformation; a cam operable to deform the structure along the planeof deformation as the cam rotates; an interrogation unit operable toreceive first and second signals corresponding to first and secondwavelengths from said at least first and second Fiber Bragg Gratings,wherein the first signal is associated with the first Fiber BraggGrating and the second signal is associated with the second Fiber BraggGrating; and a processor operable to derive a difference between thewavelengths of the first and second signals and compare the differencewith first data correlating wavelength differences to extents ofdeformation of the structure to yield a current extent of deformation.2. The sensor of claim 1 wherein the structure is further defined as aspring member having at least one substantially straight arm portion. 3.The sensor of claim 2 wherein the spring member is further defined ashaving a base portion, the at least one arm portion extending from thebase portion, and a tip at an end of the at least one arm portion distalto the base portion; wherein the cam is operable to apply deformingforce directly to the tip of the spring member.
 4. The sensor of claim 3further comprising a spiral transfer shaft secured to the cam, whereinrotation of the spiral transfer shaft causes rotation of the cam.
 5. Thesensor of claim 4 further comprising at least one bearing that supportsthe spiral transfer shaft.
 6. The sensor of claim 5 further comprising:an actuation tube housing at least a portion of a) the spiral transfershaft, and b) the at least one bearing; and a nut that encircles thespiral transfer shaft and is fixed to the actuation tube; wherein thenut forces the spiral transfer shaft to rotate in response to linearmovement of the actuation tube.
 7. The sensor of claim 6 furthercomprising an outer tube, wherein the actuation tube is locatedconcentrically within and supported by the outer tube.
 8. The sensor ofclaim 5 further comprising two bearings that support that spiraltransfer shaft.
 9. The sensor of claim 1 wherein the processor isfurther programmed to derive a current temperature of the structure. 10.The sensor of claim 1 wherein the processor is further programmed tocompare the current extent of deformation with second data correlatingextents of deformation and wavelengths for a plurality of discretetemperatures.
 11. The sensor of claim 10 wherein the processor isfurther programmed to derive a current temperature of the structure withthe second data by interpolating between first and second differentwavelengths both at the current extent of deformation and at first andsecond different temperatures based on one of the first and secondsignals.
 12. The sensor of claim 10 wherein the processor is furtherprogrammed to derive a current temperature of the structure with thesecond data by determining the difference between the wavelength of oneof the first and second signals, at the current extent of deformation,and wavelength for a plurality of discrete temperatures at the currentextent of deformation.
 13. A method of sensing comprising the steps of:directing light through a fiber and first and second Fiber BraggGratings disposed along the fiber; disposing the fiber on a structureoperable to deform such that the first and second Fiber Bragg Gratingsare positioned on opposite sides of the structure relative to a plane ofdeformation of the structure; deforming the structure along the plane ofdeformation by a rotating cam; receiving with an interrogation unitfirst and second signals each corresponding to a wavelength, wherein thefirst signal is associated with the first Fiber Bragg Grating and thesecond signal is associated with the second Fiber Bragg Grating;deriving with a processor the difference between the wavelengths of thefirst and second signals; and comparing with the processor thedifference derived in said deriving step with first data correlatingwavelength differences and extents of deformation of the structure toyield a current extent of deformation of the structure.
 14. The methodof claim 13 further comprising the step of: comparing with the processorthe current extent of deformation with second data correlating extentsof deformation and wavelengths for a plurality of discrete temperatures.15. The method of claim 14 further comprising the step of: deriving withthe processor a current temperature of the structure with the seconddata by interpolating between first and second different wavelengthsboth at the current extent of deformation and at first and seconddifferent temperatures based on one of the first and second signals. 15.The method of claim 14 further comprising the step of: deriving with theprocessor a current temperature of the structure with the second data bydetermining the difference between the wavelength of one of the firstand second signals, at the current extent of deformation, and wavelengthfor a plurality of discrete temperatures at the current extent ofdeformation.
 16. A method comprising the steps of: providing a valve andthe sensor of claim 1; operatively connecting the valve to the cam ofthe sensor; and determining a position of the valve by the sensor.
 17. Amethod comprising the steps of: providing an actuator and the sensor ofclaim 6; operatively connecting the actuator to the actuation tube ofthe sensor; and determining a stroke length of the actuator by thesensor.
 18. A method comprising the steps of: providing an inlet guidevane and the sensor of claim 1; operatively connecting the inlet guidevane to the cam of the sensor; and determining a position of the inletguide vane by the sensor.